# No.23 **Charge output type accelerometer and charge converter**

There are 2 types of accelerometers, one is the built-in preamplifier, and another is the charge output.

The charge output type is not the built-in preamplifier, thus suitable for the measurement of vibration such as impact and drop tests and can be used in higher temperature environment. When analyzing the charge signal output with the analyzer such as an FFT analyzer or Data station, it is converted into a voltage signal by a charge amplifier or charge converter and input to the analyzer.

CH-6130/6140 charge converter is easy to handle to connect to BNC connector of analyzer. While the gain is fixed, the acceleration which is smaller than the maximum operating acceleration of the charge output type accelerometer can be only measured depending on combinations. When selecting accelerometer/ charge converter, it is necessary to select the appropriate products to check the actual acceleration.

**CH-6130/6140 charge converter**

The main specifications for CH-6130/6140 charge converter is shown as follow. The differences between CH-6130 and CH-6140 are gain and price. CH-6130 is smaller gain, thus suitable for measuring large acceleration. CH-6140 is larger gain, thus suitable for measuring small acceleration. Anyway, the maximum output voltage ± 5 V causes the output to reach its peak, thus the large acceleration cannot be measured.

Table1 main specifications for CH-6130/6140

Model | CH-6130 | CH-6140 |
---|---|---|

Gain | 1.0 mV/pC | 10.0 mV/pC |

Frequency range | 5 Hz to 15 kHz (±0.5 dB) | 5 Hz to 15 kHz (±0.5 dB) |

2 Hz to 45 kHz (±3 dB) | 2 Hz to 45 kHz (±3 dB) | |

Output voltage | 10 V |
10 V |

Input referred noise | 0.05 pC(rms) or less | Max. 0.05 pC(rms) or less |

CCLD power supply | Voltage : 18 to 36V | Voltage : 18 to 36V |

Constant current : 2.0 to 20 mA | Constant current : 2.0 to 20 mA |

The appearance of CH-6130 is shown in Figure 1. The appearance of CH-6140 is same as CH-6130 except model name.

Fig.1 CH-6130 charge converter

**Combination of charge output type accelerometer and charge converter**

The voltage sensitivity when the charge-type accelerometer is used in combination with the charge converter is the value which is multiplied by the charge sensitivity of the accelerometer pC/(m/s2) and the gain of the charge converter (mV/pC). The voltage sensitivity is 0.35 mV/(m/s2) to be multiplied by the charge sensitivity of NP-2106, 0.035 pC/(m/s2) and the gain of CH-6140, 10 (mV/pC). It means 0.35 mV output against 1 m/s2.

The maximum use acceleration of charge output type accelerometer is the maximum acceleration at which the accelerometer is working correctly. Due to limitations on the maximum output voltage of the charge converter, the maximum use acceleration when combined may be smaller than the maximum use acceleration of the accelerometer itself.

The voltage sensitivity when combining NP-2106 and CH-6140 is 0.35 mV/ (m/s2), and the maximum output voltage of CH-6140 is ± 5 V, so the maximum use acceleration is V ÷ (0.35 mV/(m/s2)) = 14,285 m/s2. NP-2106’s maximum use acceleration is 100,000 m/s2, while when combined with CH-6140, the maximum use acceleration is only14,285 m/s2. If measuring larger acceleration, it is necessary to combine with CH-6130 or charge amplifier.

The input referred noise of CH-6130/6140 is less than 0.05 pC. Even if the acceleration is 0, the noise equivalent to less than 0.05 pC is output. The charge sensitivity of NP-2106, 0.035 pC/(m/s2) is 1.43 m/s2 = 0.05 pC ÷ (0.035 pC / (m/s2)) when converted to acceleration. Depending on the measurement method or how accurate the measurement value is, the value that is 10 or 30 times of the input referred noise is the estimated lower range of acceleration that can be measured. NP-2106 is the accelerometer for collision/ drop test that can measure up to 100,000 m/s2, thus it is not suitable for measuring less than a few hundreds of acceleration. As for the measurement of smaller acceleration, please use the accelerometer with large sensitivity.

The voltage sensitivity, the maximum use acceleration, input referred noise is shown in table 2, 3 when combined the charge output type accelerometer and CH-6130/6140.

The maximum use acceleration of each accelerometer when combined with CH-6140 is smaller than the maximum use acceleration of single unit. The maximum use acceleration of NP-2710, NP-2910, NP-2810, NP-2120 when combined with CH-6130 is smaller than the maximum use acceleration of single unit. When measuring larger acceleration than the maximum use acceleration when combined, please use charge amplifier such as CH-1200A.

Table 2 Characteristics of combination of charge converter and CH-6130

Charge output type accelerometer | Characteristics when combined with CH-6130(1.0mv/pc) | |||||
---|---|---|---|---|---|---|

Model | Futures | Charge sensitivity(typ.) | Max use acceleration | Charge sensitivity(typ.) | Max use acceleration | Input referred noise |

NP-2106 | Ultra-compact and light weighted | 0.035 pC/(m/s^{2}) |
100,000 m/s^{2} |
0.035 mV/(m/s^{2}) |
100,000 m/s^{2} |
1.43 m/s^{2} |

NP-2110 | Compact and light weighted | 0.16 pC/(m/s^{2}) |
10,000 m/s^{2} |
0.16 mV/(m/s^{2}) |
10,000 m/s^{2} |
0.31 m/s^{2} |

NP-2710 | Compact, high-temperature | 0.306 pC/(m/s^{2}) |
22,600 m/s^{2} |
0.306 mV/(m/s^{2}) |
16,339 m/s^{2} |
0.16 m/s^{2} |

NP-2910 | Compact, general purpose usage | 0.3 pC/(m/s^{2}) |
50,000 m/s^{2} |
0.3 mV/(m/s^{2}) |
16,667 m/s^{2} |
0.17 m/s^{2} |

NP-2810 | Compact | 1.2 pC/(m/s^{2}) |
20,000 m/s^{2} |
1.2 mV/(m/s^{2}) |
4,166 m/s^{2} |
0.042 m/s^{2} |

NP-2120 | General purpose usage | 5 pC/(m/s^{2}) |
8000 m/s^{2} |
5 mV/(m/s^{2}) |
1000 m/s^{2} |
0.010 m/s^{2} |

NP-2560 | Light weight tri-axial | 0.04 pC/(m/s^{2}) |
25,000 m/s^{2} |
0.04 mV/(m/s^{2}) |
25,000 m/s^{2} |
1.25 m/s^{2} |

** **

Table 3 Characteristics of combination of charge converter and CH-6140

Charge output type accelerometer | Characteristics when combined with CH-6130(1.0mv/pc) | |||||
---|---|---|---|---|---|---|

Model | Futures | Charge sensitivity(typ.) | Max use acceleration | Charge sensitivity(typ.) | Max use acceleration | Input referred noise |

NP-2106 | Ultra-compact and light weighted | 0.035 pC/(m/s^{2}) |
100,000 m/s^{2} |
0.35 mV/(m/s^{2}) |
14285 m/s^{2}^{} |
1.43 m/s^{2} |

NP-2110 | Compact and light weighted | 0.16 pC/(m/s^{2}) |
10,000 m/s^{2} |
1.6 mV/(m/s^{2}) |
3125 m/s^{2}^{} |
0.31 m/s^{2} |

NP-2710 | Compact, high-temperature | 0.306 pC/(m/s^{2}) |
22,600 m/s^{2} |
3.06 mV/(m/s^{2}) |
1633 m/s^{2}^{} |
0.16 m/s^{2} |

NP-2910 | Compact, general purpose usage | 0.3 pC/(m/s^{2}) |
50,000 m/s^{2} |
3 mV/(m/s^{2}) |
1666 m/s^{2}^{}^{} |
0.17 m/s^{2} |

NP-2810 | Compact | 1.2 pC/(m/s^{2}) |
20,000 m/s^{2} |
12 mV/(m/s^{2}) |
416 m/s^{2}^{}^{} |
0.042 m/s^{2} |

NP-2120 | General purpose usage | 5 pC/(m/s^{2}) |
8000 m/s^{2} |
50 mV/(m/s^{2}) |
100 m/s^{2} |
0.010 m/s^{2} |

NP-2560 | Light weight tri-axial | 0.04 pC/(m/s^{2}) |
25,000 m/s^{2} |
0.4 mV/(m/s^{2}) |
12500 m/s^{2}^{} |
1.25 m/s^{2} |

** **

**Summary**

The accelerometer has the maximum allowable acceleration, and acceleration exceeding that value cannot be measured. When using a charge output type accelerometer in combination with the charge converter, the output voltage of the charge converter is limited, thus the maximum measurable acceleration may be smaller than the maximum acceleration of the accelerometer alone.

In this article, the maximum allowable acceleration values when combining the charge output type accelerometer and charge converter are referred. Also, the charge sensitivity and input referred noise are referred.

It is necessary to select the accelerometer and the charge converter according to the magnitude of acceleration generated by the measurement object. Not the charge converter but the charge amplifier is sometimes necessary.

For measuring object first time, the magnitude of the acceleration is unknown. There may be a case where the acceleration is much larger than expected in the impact / drop test etc. In order to confirm the actual acceleration in such cases, we are renting the demo or demonstrating the products. Please contact the nearest sales office.

(YK)